Morphologic, stratigraphic and morphometric investigations of valley networks in eastern Libya Montes, Mars: Implications for the Noachian/Hesperian climate change

"Dendritic valley networks" and broad "longitudinal valleys" are widespread within the eastern Libya Montes region. The morphologic characteristics of the studied dendritic valleys, especially their occurrence at local summits, provide evidence for the initial formation by surface runoff due to atmospheric precipitation whereas the longitudinal valley systems are thought to have been formed and modified by groundwater-induced processes. Our crater counts and model ages show, that the dendritic valley networks (average age similar to 4.0 Ga) formed earlier than the longitudinal valley systems (average age similar to 3.7 Ga). Both fluvial forms indicate a temporal change of the erosive environment at the Noachian/Hesperian transition at similar to 3.8 Ga. Our results suggest that the fluvial activity within our investigation area lasted for similar to 800 My, from similar to 4.1 Ga to similar to 3.3 Ga. The dendritic valleys show valley densities between 0.12 km(-1) and 0.57 km(-1), indicative of highly mature and integrated valley systems that we interpret to be characteristic of warmer and wetter conditions on Mars. (C) 2009 Elsevier B.V. All rights reserved.